Knowledge of the volume of fluid (such as a liquid, gas, or any other form of matter capable of flowing in accordance with principles of fluid dynamics) that flows from a source to a destination is desirable and advantageous in many applications. In many cases, the volume of fluid in a destination container may be directly measurable using standard fluid volume measurement techniques. For example, a container at the destination may be provided with a window through which the level of the fluid is visible. A window, dipstick, or container surface may include graduation marks for quantitatively indicating the fluid level in the container. A float or sensor may cooperate with an indicator to indicate the level of fluid in the container. If the dimensions of the destination container are known, or if the graduation marks or indicator have been calibrated, the volume of fluid in the destination container may be inferred. Measuring the volume of fluid in the destination container before and after the flow indicates the volume of fluid that flowed from source to volume.
In some cases, the volume of fluid at the destination cannot be measured directly. For example, there may not be access to fluid at the destination. As another example, or fluid may flow out of the destination container concurrently with the inflow of fluid from the source, or concurrently flow into the destination from another source. In some such cases, the volume of fluid flowing from the source into the destination may be deduced from a change of the volume of fluid in the source. For example, flow of fluid may be limited to a conduit that connects the source to the destination. Thus, a reduction in a measured volume of fluid in a source container may indicate that a similar volume of fluid has flowed into the destination.
In some applications, neither the volume of fluid flowing out of the source nor the volume of fluid arriving at the destination is directly measurable. For example, direct access to the fluid at the source may not be possible. If the fluid in the source is replenished or flows outward to another destination concurrently with flow from the source to the destination, the amount of flow from the source to the destination may not be deduced from the change in fluid volume at the source.
In some cases, therefore, the volume of fluid flowing from the source to the destination may be deduced from a measured flow between the source and the destination. For example, a conduit connection the source to the destination may include a flowmeter. A flowmeter may indicate the rate of flow of the fluid past a point of the conduit as a volume of fluid per unit time. Integrating the flow rate over a period of time yields the volume that flowed past the flowmeter during that period of time.
A rate of flow may refer to any of several related quantities. For example, a rate of flow may refer to a velocity of a fluid (e.g. displacement of a volume of the fluid per unit time) or to a mass or volume of fluid flowing past a given point or plane per unit time.
Flowmeters have been designed for different circumstances and are based on various principles. For example, various flowmeters have been described that are based on the flowing fluid exerting a mechanical force on an object (e.g. by Head et al. in U.S. Pat. No. 4,051,723 and Anderson et al. in U.S. Pat. No. 4,195,522), on Coriolis forces in the fluid (e.g. van der Pol et al. in U.S. Pat. No. 6,301,974), on electrical properties of the flowing fluid (e.g. Rottenberg et al. in EP 0077413), and on ultrasound (e.g. Schaffer et al. in US 2005/005709).
Flowmeters have been described that are based on induced pressure differences within the fluid. For example, a pressure difference may be introduced in a flowing fluid by a constriction in part of the conduit through which the fluid is flowing. Examples of flowmeters based on pressure differences include Venturi tubes and pipes with orifices. Such a pressure difference may be a function of the flow rate of the fluid. One or more pressure gauges may measure the pressure or pressure differences. A flow rate may then be deduced from the measured pressures. For example, at a point in the conduit, part of the fluid may be directed through a Venturi tube while part of the liquid bypasses the Venturi tube (e.g. Takamura et al. in U.S. Pat. No. 3,924,467, Kohmura et al. in US 2002/166376, and Shiba in U.S. Pat. No. 3,693,437).
Different types of pressure gauges have been described for measuring the pressure of a fluid. For example, pressure gauges have been described in which a diaphragm or membrane is moved by differing pressures on either side of it. For example, one side of the diaphragm may be exposed to a fluid whose pressure is to be measured, and the other to a fluid at a reference pressure. The diaphragm or other components of the diaphragm pressure gauge may be connected to a suitable electrical circuit. Motion of the membrane under the influence of the pressure difference may be sensed as a change in capacitance (e.g. as described by Spaulding in U.S. Pat. No. 2,667,786 or by Slavin et al. in U.S. Pat. No. 3,993,939).
For example, it may be desirable to know the milk intake of a child during breastfeeding. In this case, direct measurement of the volume of milk taken in by the child or the volume of milk output by the breast, may be difficult. Therefore, devices have been described for measuring or estimating the milk intake. For example, a device for measuring the flow rate of milk using Doppler ultrasound may be placed around the breast during breastfeeding (e.g. as described by Kolberg et al. in US 2009/054771). Other described devices include a cap through which milk may flow may be placed over the nipple during breastfeeding. For example, a cap that covers the nipple may divert some of the milk flowing through it to an indicator whose level is related to the volume of the milk that flowed through the cap (e.g. as described by Dahan et al. in US 2005/177099 and in US 2006/226108). Nipple caps have also been described that measure the flow rate or volume of milk passing through them using various techniques (e.g. by Rosenfeld in U.S. Pat. No. 5,827,191, Ezra et al. in US 2008/167579, and Shemesh et al. in US 2008/039741).
It is an object of the present invention to provide a flowmeter for measuring the flow of a fluid, applicable to a variety of flow applications.
Other aims and advantages of the present invention will become apparent after reading the present invention and reviewing the accompanying drawings.